1. FIELD OF THE INVENTION
The present invention relates to an ignition device for an internal combustion engine which is capable of preventing ignition spark from occuring when the engine rotates in the direction opposite to a predetermined direction.
2. 2. DISCUSSION OF BACKGROUND
One device of this type as is known in the art is shown in FIG. 3. In FIG. 3, a reference numeral 1 designates a rotor which is driven by the engine (not shown) in the direction of an arrow 2. The rotor has a predetermined size of projection formed on the circumferential surface between positions A and B.
A pickup 3 is arranged to be opposite to the rotor 1 so as to work as an angular position sensor. The pickup 3 detects a first crank angle and a second crank angle. When the forward end A of the projection located on the side of the rotational direction passes over the pickup 3, the pickup outputs a positive voltage indicative of the first angular signal. On the other hand, when the rearward end B of the projection located on the side of the anti-rotational direction passes the pickup 3, the pickup outputs a negative voltage indicative of the second angular signal.
The pickup 3 has a coil connected to a first diode 4 and a second diode 5 so that the first and the second diode perform a distinction between the positive and the negative voltage.
The first diode 4 has the anode connected to the coil of the pickup 3 and the cathode connected to the set input terminal of a flip-flop (hereinbelow, referred to as FF) 6.
The second diode 5 has the cathode connected to the coil of the pickup 3 and the anode connected to the reset input terminal of the FF 6.
As a result, positive waves in the output voltage of the pickup 3 pass through the first diode 4 and come into the set input terminal of the FF 6 so as to cause the flip-flop to set to the 1 state. On the other hand, negative waves in the output voltages of the pickup 3 pass through the second diode 5 and come into the reset input terminal of the FF 6 so as to cause the FF 6 to reset to the 0 state.
A transistor 7 with the emitter grounded has the base connected to the output terminal Q of the FF 6 and the collector connected to one end of the primary winding of an ignition coil 8.
The other end of the primary winding of the ignition coil 8 is connected to a battery 8 through a key switch 9. The high-tension cable of the ignition coil 8 is connected to an ignition plug 11.
FIG. 5 is a drawing of waveforms to help explain the operation of the system as shown in FIG. 3. The output voltage a of the pickup 3 is shown in FIG. 5 at (A). The set input voltage b of the FF 6 is shown in FIG. 5 at (B). The reset input voltage c of the FF 6 is shown in FIG. 5 at (C). The output voltage d at the output terminal Q of the FF 6 is shown in FIG. 5 at (D). The primary current e of the ignition coil 8 is shown in FIG. 5 at (E). The secondary voltage f of the ignition coil 8 is shown in FIG. 5 at (F). That is to say, the waveforms at the positions a-f in FIG. 3 are shown in FIG. 5 at (A).varies.(F).
In operation, when the rotation of the rotor 1 causes the forward end A of the projection to become opposite to the pickup 3, the positive voltage is induced in the pickup coil based on the principal of electromagnetic induction. The induced positive voltage causes the FF 6 to set to the 1 state through the first diode 4. As a result, the output terminal Q of the FF 6 becomes a high level (hereinbelow, referred to as "1").
The "1" signal from the output terminal Q allows the transistor 7 to conduct so that the primary current of the ignition coil 8 starts flowing at the moment.
When the rotation of the rotor 1 progresses, the rearward end B of the projection becomes opposite to the pickup 3. At this time, the negative voltage is induced in that pickup coil. The induced negative voltage causes the FF 6 to reset through the second diode 5. As a result, the output terminal Q of the FF 6 becomes are a low level (hereinbelow, referred to as "0". The "0" signal from the output terminal Q cut off the transistor 7 to interrupt the primary current of the ignition coil 8. At the interrupting moment, a high voltage is induced in the secondary winding of the ignition coil 8 to produce an ignition spark at the ignition plug 11. Ignition sparks subsequent to the ignition spark as just mentioned are cyclically produced in accordance with the progressing rotation of the rotor 1 as shown in FIG. 5.
Although there is no inconvenience in the normal operation, there is inconvenience in the normal operation, such as the presence of a wide range of fluctuation in the rotation of the engine at the time of starting the engine, and the occurence of the reverse rotation of the engine caused by failing to overcome counter-torque, as follows.
For example, imagine that the system is constructed so that the forward end A of the rotor 1 detects BTDC (before top dead center) 35 degrees and the rearward end B detects BTDC 5 degrees (for example, the BTDC 5 degrees correspond to the initial ignition timing, the BTDC 35 degrees correspond to the operation commencement time in an electric advance type of ignition timing operation circuit. Both figure are quite usual.).
Assuming that the rotor 1 was in the position as shown in FIG. 3 before starting the engine and the starting operation starts. The rotor 1 starts to be gradually rotated by external force such as a starter (not shown) in the direction indicated by the arrow 2 (hereinbelow, referred to as "the forward direction") or "forward rotation"). When the forward end A becomes opposite to the pickup 3, the positive voltage is induced in the pickup coil to cause the FF 6 to set to the 1 state. As a result, the primary current starts through the ignition coil 8.
After that, when the rotor continues the forward rotation, the engine approaches the top center in its compression stroke to receive the counter-torque being increased. Under bad conditions wherein the battery is almost discharged or the outside temperature is low, the engine sometimes fails to rotate beyond the top dead center in the compresson stroke and is reversed on the way.
In addition, there is a possibility of stopping the starting operation (turning off the starter switch) before the engine has fired. In such case, the driving force given to the engine goes down to 0, accelerating the reverse rotation by the counter-torque.
In either case, the rotor 1 which has progressed the forward rotation up to around, for example, the BTDC 15 degrees, starts the reverse rotation from that point. In the course of the reverse rotation of the rotor, the change in the magnetic field caused when the forward end A passes over the pickup 3 is the same as that caused when the rearward end B becomes opposite to the pickup 3 in the course of the forward rotation of the rotor. As a result, when the forward end A passes over the pickup 3 in the course of the reverse rotation, the negative voltage is induced in the pickup coil.
For the reasons, the induced negative voltage causes the FF 6 to reset to interrupt the primary current in the ignition coil 8 so that the ignition spark is produced at that moment.
In other wards, the ignition spark is produced at the position of BTDC 35 degrees in terms of engine crank angle. That causes the compressed and mixed air and gasoline vapor to be ignited so as to add further reverse rotation torque to the engine being reversed. As a result, the reverse rotation of the engine is acceleratated, and the engine is sometimes destroyed.
Since the conventional ignition device for an internal combustion engine is constructed as mentioned above, there is a disadvantage in that the ignition spark can be produced even in the reverse rotation of the engine to destroy the engine.